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THE TRANSPORTATION OF LIQUIDS TABLE 1.—SPEEDS OF PUMPS
111
	Direct-acting pumps								Crank and
									flywheel
Stroke	Piston pumps, plunger and ring pumps, outside packed pumps up to 300 Ib. pressure				Pressure-pumps, wet vacuum pumps	Boiler-feed p. and pumps for handling thick liquids	Direct-acting Water motor driven pumps	Geared power pumps	pumps also belt    or direct-driven power pumps
	Simple cyl. and Compound		Triple expansion						
	Cycles	Feet per	Cycles	Feet per	Feet per	Feet per	Feet per	Feet per	Feet per
Inches	per minute	minute	per minute	minute	minute	minute	minute	minute	minute
3	80	40			32	20	13	32	
4	75	50			40	25	17	40	
5	72	60			48	30	20	48	
6	65	65			52	33	22	52	
8	56.2	75			60	38	25	60	
10	48	80	54	90	64	40	27	64	
12	45	90	50	100	72	45	30	72	180
15	40	100	44	110	80	50	33	80	200
18	36.7	110	40	120	88	55	37	88	220
24	30	120	32.5	130	96	60	40	96	240
30	25	125	28	140	100	63	42	100	250
36	22.5	135	25	150	108	68	45	108	270
48	18	145	20	160	116	73	48	116	290
60	15 1	150	17	170	120	75	50	120	300
For definition of cycle, see p. 135.
FIG. 3.-^-Sections of compound steam end showing cross exhaust and floating sleeve.
The steam in expanding from the high-pressure cylinder into the low-pressure cylinder is reduced in pressure, resulting in a declining propulsion curve and short stroking. By opening the cross exhaust valve c the size of the intermediate space is doubled and, therefore, the high-pressure-exhaust steam expands to a lower pressure when flowing into the intermediate space. The low-pressure initial pressure is thus reduced while the terminal pressure remains the same. This results in a more uniform back pressure in the intermediate space, and a more nearly straight propulsion curve, which is the aim of the designer of a direct-acting pump, since the load line of a pump is straight. In this way the stroke is lengthened but at the expense of power.
Single pumps are also tandem-compounded, but either the ratio of cylinders is less than for a duplex pump, due to the lack of the cross exhaust or a large receiver is inserted which serves a similar purpose as the cross exhaust.
Triple expansion direct-acting steam pumps have three cylinders on each side, arranged tandem, viz.: one high-pressure, one intermediate-pressure, one low-